The only players who have a chance of catching up to Fed are the rest of the top 4 - each of which has a real chance.
Andy Murray would pretty much have to win Paris and YEC to best Fed, but I would consider him to be the most potent hard court threat at this moment so I don't see that as impossible - injury status excepted.
Novak is defending last year's Paris Open and he's healthy, so I wouldn't consider him to be an impossibility.
Rafa is the world's best player, so he could overtake Fed easily in those 2 tournaments.
Rog is the world's best hardcourt player mostly on the strength of his AO win. And, yeah, AO happened in '10, but I think alot of us are a little bit iffy on which season AO should count toward.